Control of noise is important as sensitivity to the perceived environment increases. Thus, such rotating machinery as gas turbine engines and in particular such engines used for aircraft operations are constantly reviewed in terms of noise targets. Clearly, these noise targets are increasingly stringent with an ongoing objective to reduce environmental and where appropriate aircraft cabin noise. Nevertheless, rotating machinery by its nature will create noise and in particular so called buzz saw noise generated by the rotating fan assembly.
Buzz-saw noise occurs when the fan assembly operates with the rotor blade tips—or a lower span of the rotor blades—at sonic or supersonic Mach numbers, i.e. when Mr>1. Where Mr is the resultant rotor relative tip Mach number associated with the inlet fluid flow Mach number in the stationary frame of reference, Ma and the effective blade tip circumferential Mach number Mt. i.e. Mr=√(Mt2+Ma2). Where Mach no.=velocity/speed of sound. The shock waves, generated by the rotor assembly will sweep over the outer wall of the annulus and propagate upstream and out of the engine intake. When the outer annulus wall is treated with an acoustic layer the propagating noise will be attenuated and a lower sound pressure level will be experienced at an upstream plane relative to an engine with the same hardware but with no acoustic treatment.
The buzz-saw noise arises from the production of non-uniform shock waves resulting from the blade-to-blade geometric differences associated with the manufacturing tolerances of the rotor blades. If all blades were manufactured identically then no buzz-saw noise would occur and rotor only noise would occur at blade passing frequencies and its harmonics. This noise would then be observed by a stationary observer relative to the rotor assembly. However such a reduction in tolerances will significantly increase the manufacturing cost of the rotor blades. FIG. 1 below shows an example, considering blade stagger angle alone, of the effect of a) all blades being identical b) the effect of one blade stagger angle being larger than the others. The shocks in a) are all of equal amplitude and spacing around the fan disk and no buzz-saw noise results whilst the shocks in b) are not equal and buzz-saw noise is generated. The diagram refers to an aerodynamic condition at part speed where the shocks become “detached”. At this condition the shock non-uniformities for case b) are in fact known to be proportional to the difference in stagger angles. So that in b) Sn+1∝Θn+1−Θn (equation 1) where Sn+1 is the shock strength on the n+1 th blade, Θn+1 is the stagger angle of the n+1 th blade and Θn is the stagger angle on the n th blade. At speeds closer to design the shocks may become “swallowed”. In this case the shocks will depend upon the stagger angle.
At either condition the shocks will also have a dependence upon other blade-to-blade geometric differences such as thickness, camber, lean and leading edge blade angle. The harmonic frequencies of the buzz-saw noise are those of the harmonics of the disc rotational frequency of the fan called here engine orders. The invention is effective for either part speed conditions where the shocks are “detached” or at design conditions where the shocks are “swallowed”.
From the above it will be appreciated that the principal problem relates to the non-uniformity of the rotating assembly geometry creating disparities in the regularity of shocks at the rotational speeds defined. These disparities with respect to the adjacent casing or duct wall cause the associated buzz-saw noise. The use of acoustic wall treatments to attenuate noise levels is inhibited by the variable nature of the frequencies of the noise as well as the nature of the acoustic wall treatment, which disproportionately attenuates noise at different frequencies.
Acoustic duct liner design for frequency and attenuation is usually compromised for different specific noise sources such as broadband noise or specific aircraft conditions such as approach, take-off or cruise. Therefore the attenuation-frequency spectrum characteristic of the duct liner are usually far from ideal for the buzz-saw noise source considered here. In such circumstances, previously greater effort has been placed upon improving blade manufacturing tolerances and assembly accuracy. Nevertheless, there are commercial as well as practical limits to such approaches, i.e. more accurately manufactured and assembled blades will greatly increase costs.